The invention relates to a device for operating a linear lambda probe of an internal combustion engine.
Legislators are using tax incentives to promote the development of motor vehicles with increasingly low emissions of pollutants and fuel consumption. In spark ignition engines with stoichiometric mixture formation (λ=1), this has led to the development of SULEV vehicles (Super Ultra Low Emission Vehicles) with extremely low emissions.
In order to save fuel, engines with direct HPDI (High Pressure Direct Injection) fuel injection are currently being developed and launched onto the market. The fuel is injected directly into the combustion chamber here at an increased pressure (for example 150 bar). The preparation of the mixture which it makes possible can vary between rich, stoichiometric and lean. For the partial load mode of the engine, a lean mixture formation provides appreciable consumption advantages.
Both developments require a significantly more precise regulation of the mixture than is possible with the lambda probes (binary step-change probes) which are customary today. These probes have an extremely restrictive measuring range about λ=1 and are therefore unsuitable for measurements in the lean operating mode λ>1.
For this reason, lambda probes with an extended linear measuring range, which are referred to as linear lambda probes, are now being increasingly used.
Binary step-change probes have a pair of electrodes which are separated by a zircon ceramic, which acts as an electrolyte at high temperatures. One electrode is located in the exhaust gas stream here, and the other in air. When there is a different concentration of oxygen between the air and the exhaust gas, a voltage value which is determined by the Nernst diffusion equation is produced between the electrodes. Typical values of this voltage are approximately 200 mV in air, approximately 450 mV when λ=1 and approximately 800 mV in a rich mixture. In the region surrounding λ=1, the oxygen concentration changes by several powers of ten, which is manifested in a sudden change in the probe voltage in this region.
The linear lambda probe is of complex design. It has two pairs of electrodes and one measuring chamber which is connected to the exhaust gas stream via a diffusion barrier. The first pair of electrodes is arranged between the measuring chamber and air and is used—similarly to the step-change probe—to measure the oxygen concentration in the measuring chamber. The second pair of electrodes is arranged between the measuring chamber and the exhaust gas stream. When a current of a corresponding polarity is applied, said pair of electrodes permits oxygen ions to be pumped out of the measuring chamber or into it. Designation: pumping electrodes.
In this way it is possible to generate a dynamic equilibrium between the flow of oxygen through the diffusion barrier and the flow of oxygen ions through the pair of pumping electrodes. The oxygen concentration in the measuring cell which can be determined using the measuring electrodes is suitable as a control criterion here. A preferred value is, for example, 450 mV for λ=1. The pumping current Ip which flows in this case is therefore a measure of the oxygen concentration in the exhaust gas (and also of λ after numerical conversion).
The relation between the oxygen concentration in the exhaust gas and the pumping current is influenced by several probe parameters. For fabrication reasons, the dynamic resistance of the diffusion barrier fluctuates somewhat. This would result in a deviation in the transformation ratio (amplification error). During fabrication this is compensated by measuring and inserting a calibrating resistor Rc into the probe plug.
The calibrating resistor permits the pumping current in the following evaluation circuit to be scaled, which again compensates the transformation ratio.
Furthermore, the dynamic resistance of the diffusion barrier has a temperature dependence, which in turn leads to errors in the transmission ratio. This is counteracted by measuring the probe temperature and controlling it by means of a heating element which is installed in the probe. For reasons of cost, a separate thermal element is dispensed with here. Instead, the highly temperature-dependent internal resistance of the probe is measured.
Hitherto, the application of the linear lambda probe was restricted to the noncharged, stoichiometric operation (Pa=1 bar, λ=1) of the engine. Correspondingly, only small pumping currents were also necessary to maintain the equilibrium (λ=1) in the measuring cell (|Ip|<˜2.5 mA).
For lean engines, operation up to λ=4 is provided for, which requires a drastically increased pumping current. When operating in a supercharged engine (turbo), an exhaust gas pressure of up to 2 bar is obtained. The pressure sensitivity of the probe leads to a further increase in the maximum necessary pumping current to ±12 mA. This is possible only to a partial extent with evaluation circuits which are currently on the market.
A known evaluation circuit is illustrated in FIG. 1 and will be described in more detail below.
This circuit has certain disadvantages:
When the evaluation circuit is supplied with a supply voltage Vcc=5 V which is already generally present, a mid-voltage Vm of approximately 2.5 V is obtained. The voltage chain which is present at the probe is then:Vm<|Rc*Ip+Vp|+Vsat;                Rc=30 to 100Ω=entire calibrating resistance (dependent on manufacturer),        Vp=−350 to +450 mV; polarization voltage of the pumping cell,        Vsat=100 to 200 mV; saturation voltage of the pumping current source P.        
This limits the maximum possible pumping current Ip to <10 mA, and therefore does not correspond to the requirements (Ip=±12 mA).
Alternatively, a higher supply voltage of the circuit could be used, for example 8 V. However, this would have the disadvantage that an additional expensive voltage controller would be necessary and the circuit would no longer function when the battery voltage is low (<8.5 V). (the minimum acceptable battery voltage is defined as Vbatmin=6 V).
A common mode signal (Vm±2 V) is superimposed on the pumping current Ip. Due to the finite common mode suppression of real integrated amplifiers (for example 65 dB), the measurement is falsified by up to ±0.3%.
In addition, the polarization voltage of the pumping cell (−350 mV when λ<1) results in a zero point error ΔIp of approximately 5 μA. As the pumping current Ip is the primary measurement signal of the oxygen probe, these errors directly affect the overall precision of the probe measurement signal. This restricts the precision of the lambda control and thus constitutes a significant problem.